The first electrocardiogram (ECG) was invented by a Dutch physiologist, Willem Einthoven, in 1903, who used a string galvanometer to measure the electrical activity of the heart. Generations of physicians around the world have since used ECGs, in various forms, to diagnose heart problems and other potential medical concerns. Although the basic principles underlying Dr. Einthoven's original work, including his naming of various waveform deflections (Einthoven's triangle), are still applicable today, ECG machines have evolved from his original three-lead ECG, to ECGs with unipolar leads connected to a central reference terminal starting in 1934, to augmented unipolar leads beginning in 1942, and finally to the 12-lead ECG standardized by the American Heart Association in 1954 and still in use today. Further advances in portability and computerized interpretation have been made, yet the electronic design of the ECG recording apparatuses has remained fundamentally the same for much of the past 40 years.
Essentially, an ECG measures the electrical signals emitted by the heart as generated by the propagation of the action potentials that trigger depolarization of heart fibers. Physiologically, transmembrane ionic currents are generated within the heart during cardiac activation and recovery sequences. Cardiac depolarization originates high in the right atrium in the sinoatrial (SA) node before spreading leftward towards the left atrium and inferiorly towards the atrioventricular (AV) node. After a delay occasioned by the AV node, the depolarization impulse transits the Bundle of His and moves into the right and left bundle branches and Purkinje fibers to activate the right and left ventricles.
During each cardiac cycle, the ionic currents create an electrical field in and around the heart that can be detected by ECG electrodes placed on the skin. Cardiac electrical activity is then visually represented in an ECG trace by PQRSTU-waveforms. The P-wave represents atrial electrical activity, and the QRSTU components represent ventricular electrical activity. Specifically, a P-wave represents atrial depolarization, which causes atrial contraction.
P-wave analysis based on ECG monitoring is critical to accurate cardiac rhythm diagnosis and focuses on localizing the sites of origin and pathways of arrhythmic conditions. P-wave analysis is also used in the diagnosis of other medical disorders, including imbalance of blood chemistry. Cardiac arrhythmias are defined by the morphology of P-waves and their relationship to QRS intervals. For instance, atrial fibrillation (AF), an abnormally rapid heart rhythm, can be confirmed by the presence of erratic atrial activity or the absence of distinct P-waves and an irregular ventricular rate. Atrial flutter can be diagnosed with characteristic “sawtooth” P-waves often occurring twice for each QRS wave. Some congenital supraventricular tachycardias, like AV node re-entry and atrioventricular reentrant tachycardia using a concealed bypass tract, are characterized by an inverted P-wave occurring shortly after the QRS wave. Similarly, sinoatrial block is characterized by a delay in the onset of P-waves, while junctional rhythm, an abnormal heart rhythm resulting from impulses coming from a locus of tissue in the area of the AV node, usually presents without P-waves or with inverted P-waves within or shortly before or after the QRS wave. Also, the amplitudes of P-waves are valuable for diagnosis. The presence of broad, notched P-waves can indicate left atrial enlargement or disease. Conversely, the presence of tall, peaked P-waves, especially in the initial half, can indicate right atrial enlargement. Finally, P-waves with increased amplitude can indicate hypokalemia, caused by low blood potassium, whereas P-waves with decreased amplitude can indicate hyperkalemia, caused by elevated blood potassium.
Cardiac rhythm disorders may present with lightheadedness, fainting, chest pain, hypoxia, syncope, palpitations, and congestive heart failure (CHF), yet rhythm disorders are often sporadic in occurrence and may not show up in-clinic during a conventional 12-second ECG. Some atrial rhythm disorders, like atrial fibrillation, are known to cause stroke, even when intermittent. Continuous ECG monitoring with P-wave-centric action potential acquisition over an extended period is more apt to capture sporadic cardiac events that can be specifically identified and diagnosed. However, recording sufficient ECG and related physiological data over an extended period remains a significant challenge, despite an over 40-year history of ambulatory ECG monitoring efforts combined with no appreciable improvement in P-wave acquisition techniques since Dr. Einthoven's original pioneering work over a 110 years ago.
Electrocardiographic monitoring over an extended period provides a physician with the kinds of data essential to identifying the underlying cause of sporadic cardiac conditions, especially rhythm disorders, and other physiological events of potential concern. A 30-day observation period is considered the “gold standard” of monitoring by some, yet a 14-day observation period is currently deemed more achievable by conventional ECG monitoring approaches. Realizing a 30-day observation period has proven unworkable with existing ECG monitoring systems, which are arduous to employ; cumbersome, uncomfortable and not user-friendly to the patient; and costly to manufacture and deploy. An intractable problem is the inability to have the monitoring electrodes adhere to the skin for periods of time exceeding 5-14 days, let alone 30 days. Still, if a patient's ECG could be recorded in an ambulatory setting over a prolonged time periods, particularly for more than 14 days, the chances of acquiring meaningful medical information and capturing an abnormal event while the patient is engaged in normal activities are greatly improved.
The location of the atria and their low amplitude, low frequency content electrical signals make P-waves difficult to sense, particularly through ambulatory ECG monitoring. The atria are located either immediately behind the mid sternum (upper anterior right atrium) or posteriorly within the chest (left atrium), and their physical distance from the skin surface, especially when standard ECG monitoring locations are used, adversely affects current strength and signal fidelity. Cardiac electrical potentials measured from the classical dermal locations have an amplitude of only one-percent of the amplitude of transmembrane electrical potentials. The distance between the heart and ECG electrodes reduces the magnitude of electrical potentials in proportion to the square of change in distance, which compounds the problem of sensing low amplitude P-waves. Moreover, the tissues and structures that lie between the activation regions within the heart and the body's surface further attenuate the cardiac electrical field due to changes in the electrical resistivity of adjacent tissues. Thus, surface electrical potentials, when even capable of being accurately detected, are smoothed over in aspect and bear only a general spatial relationship to actual underlying cardiac events, thereby complicating diagnosis. Conventional 12-lead ECGs attempt to compensate for weak P-wave signals by monitoring the heart from multiple perspectives and angles, while conventional ambulatory ECGs primarily focus on monitoring higher amplitude ventricular activity, i.e., the R-wave, that, comparatively, can be readily sensed. Both approaches are relatively unsatisfactory with respect to the P-wave and related need for the accurate acquisition of the P and R-wave medically actionable data of the myriad cardiac rhythm disorders that exist.
Additionally, maintaining continual contact between ECG electrodes and the skin after a day or two of ambulatory ECG monitoring has been a problem. Time, dirt, moisture, and other environmental contaminants, as well as perspiration, skin oil, and dead skin cells from the patient's body, can get between an ECG electrode's non-conductive adhesive and the skin's surface. These factors adversely affect electrode adhesion which in turn adversely affects the quality of cardiac signal recordings. Furthermore, the physical movements of the patient and their clothing impart various compressional, tensile, bending, and torsional forces on the contact point of an ECG electrode, especially over long recording times, and an inflexibly fastened ECG electrode will be prone to becoming dislodged or unattached. Moreover, subtle dislodgment may occur and be unbeknownst to the patient, making the ECG recordings worthless. Further, some patients may have skin that is susceptible to itching or irritation, and the wearing of ECG electrodes can aggravate such skin conditions. Thus, a patient may want or need to periodically remove or replace ECG electrodes during a long-term ECG monitoring period, whether to replace a dislodged electrode, reestablish better adhesion, alleviate itching or irritation, allow for cleansing of the skin, allow for showering and exercise, or for other purpose. Such replacement or slight alteration in electrode location actually facilitates the goal of recording the ECG signal for long periods of time.
Conventionally, multi-week or multi-month monitoring can be performed by implantable ECG monitors, such as the Reveal LINQ insertable cardiac monitor, manufactured by Medtronic, Inc., Minneapolis, Minn. This monitor can detect and record paroxysmal or asymptomatic arrhythmias for up to three years. However, like all forms of implantable medical device (1 MB), use of this monitor requires invasive surgical implantation, which significantly increases costs; requires ongoing follow up by a physician throughout the period of implantation; requires specialized equipment to retrieve monitoring data; and carries complications attendant to all surgery, including risks of infection, injury or death. Finally, such devices do not necessarily avoid the problem of signal noise and recording high quality signals.
Holter monitors are widely used for ambulatory ECG monitoring. Typically, they are used for only 24-48 hours. A typical Holter monitor is a wearable and portable version of an ECG that includes cables for each electrode placed on the skin and a separate battery-powered ECG recorder. The leads are placed in the anterior thoracic region in a manner similar to what is done with an in-clinic standard ECG machine using electrode locations that are not specifically intended for optimal P-wave capture but more to identify events in the ventricles by capturing the R-wave. The duration of monitoring depends on the sensing and storage capabilities of the monitor. A “looping” Holter (or event) monitor can operate for a longer period of time by overwriting older ECG tracings, thence “recycling” storage in favor of extended operation, yet at the risk of losing event data. Although capable of extended ECG monitoring, Holter monitors are cumbersome, expensive and typically only available by medical prescription, which limits their usability. Further, the skill required to properly place the electrodes on the patient's chest precludes a patient from replacing or removing the sensing leads and usually involves moving the patient from the physician office to a specialized center within the hospital or clinic.
Noise in recorded signals or other artifacts that do not reflect cardiac activity can contribute to an incorrect diagnosis of a patient. The main sources of noise in an ECG machine are common mode noise, such as 60 Hz power line noise, baseline wander, muscle noise, and radio frequency noise from equipment including pacemakers or other implanted medical devices. Such noise can contribute to an incorrect diagnosis of the patient. For example, electrical or mechanical artifacts, such as produced by poor electrode contact or tremors, can simulate life-threatening arrhythmias. Similarly, baseline wander produced by excessive body motion during an ECG procedure may simulate an ST segment shift ordinarily seen in myocardial ischemia or injury.
Current ECG over-reading software generally does not allow a user to apply an arbitrary noise filter of choice to an ECG trace; users are generally limited to a set of proprietary filters. In addition, conventional over-reading software generally fails to provide users with a way to compare the results of combinations of arbitrary noise filters, thus preventing the user from finding the most appropriate filter. This is especially relevant when trying to record the P-wave or cardiac atrial signal. Further, the interpretation of the ECG is conventionally left entirely to the user, such as a technician or a doctor, and the speed with which a patient can receive some interpretation results of his or her ECG depends entirely on when the user can get to that patient's ECG and how much time the interpretation consumes. In an environment where medical personnel resources are scarce, the interpretation may take an excessively long time.
Further, in addition to ECG signal acquisition and processing, significant challenges exist in regards to the storage of the results of acquisition and processing and making such results quickly available to only authorized parties, such as the patient or the patient's physician. Multiple laws govern the safeguarding of electronic patient records. For example, in the United States, the governing law includes Health Insurance Portability and Accountability Act (HIPAA) while in the European Union the law includes the European Union's Data Protection Directive. In particular, such laws, and HIPAA in particular, focus protection on individually identifiable health information, information that can be tied to a particular patient. Such patient identifying information can include information on the patient's physical or mental health, provision of care to the patient, payment for provision of health care, and identifying information such as name, address, birth date, and social security number. Disclosure of such information in breach of the applicable laws can incur significant penalties. Considering that the disclosure can happen through ways as diverse as a hack of a database containing the records, personnel error, and loss of access information for the database, significant potential for an illegal disclosure exists with conventional record storage techniques.
Therefore, a need remains for a way to facilitate real-time, interactive processing of an ECG.
An additional need exists to accelerate ECG over-reading.
A still further need remains for a way to securely store and provide access to results of ECG analyses and other identifying information.
A further need remains for a low cost extended wear continuously recording ECG monitor attuned to capturing low amplitude cardiac action potential propagation for arrhythmia diagnosis, particularly atrial activation P-waves, and practicably capable of being worn for a long period of time, especially in patient's whose breast anatomy or size can interfere with signal quality in both women and men.